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Instead of measuring water temperature, they determined the ratio of noble gases in the atmosphere, which are in direct relation to the ocean’s temperature. (File Photo)

The modern global ocean’s average temperature is 3.5 degrees Celsius, say scientists who have found a ‘ground-breaking’ approach to measure the average temperature of the ocean. Determining changes in the average temperature of the entire world’s ocean has proven to be a nearly impossible task due to the distribution of different water masses, researchers said.

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Each layer of water can have drastically different temperatures, so determining the average over the entirety of the ocean’s surface and depths presents a challenge, they said. Now, researchers at the University of California San Diego in the US have been able to bypass these obstacles by determining the value indirectly.

Instead of measuring water temperature, they determined the ratio of noble gases in the atmosphere, which are in direct relation to the ocean’s temperature. “This method is a radically new way to measure change in total ocean heat,” said Jeff Severinghaus, a geoscientist at University of California San Diego.

“It takes advantage of the fact that the atmosphere is well-mixed, so a single measurement anywhere in the world can give you the answer,” said Severinghaus. In the study published in the journal Nature, the scientists measured values of the noble gases argon, krypton, and xenon in air bubbles captured inside ice in Antarctica.

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As the oceans warm, krypton and xenon are released into the atmosphere in known quantities. The ratio of these gases in the atmosphere therefore allows for the calculation of average global ocean
temperature. The study determined that the average global ocean temperature at the peak of the most recent ice age was 0.9 degrees Celsius.

The modern ocean’s average temperature is 3.5 degrees Celsius. The incremental measurements between these data points provide an understanding of the global climate never before possible, researchers said. Measurements were taken from ice samples collected during the West Antarctic Ice Sheet (WAIS) Divide coring project.

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Over the course of six field seasons in Antarctica, a drill removed ice in cylindrical samples 3.7 meters in length. The final sample was taken at a depth of 3,405 metres in 2011. This record spans nearly 100,000 years and the age of the layers can be determined to within 50 years. Earth’s atmosphere mixes on a scale of weeks to months, so a measurement of these air bubbles gives what is essentially a global average.

Scientists focused on samples 8,000 to 22,000 years old, and collected data in increments averaging 250 years in resolution.